The present embodiments relate to deflecting a beam of electrically charged particles onto a curved particle path.
Particle therapy includes accelerating ions of hydrogen (protons), carbon (C12) or other elements to high velocities (equivalent to energies of 50-500 MeV/nucleon) and directing the ions or other elements onto tumor tissue that is to be treated. The depth of penetration into the tissue can be set by varying the particle energy. The ions are generated and accelerated in a part of the system, which because of the part's size, that is stationary. The part of the system is immovable. The tumor may be irradiated from different directions. Accordingly, a movable magnet system for guiding and deflecting the ion beam may be provided. The magnet system may be adjustable in strength in order to enable an alignment with different particle energies, something which can be implemented effectively by electromagnets.
A magnet system that is able to rotate around the patient may be a gantry. A gantry comprises a system of magnets for deflecting and focusing ions of different energy and the mechanisms for mounting and rotating. The ion beam having a small beam diameter of a few millimeters exits the stationary generation and accelerator system and is injected into the gantry's magnet system, which is rotatable around the beam axis. In the gantry's magnet system, the beam experiences a deflection out of the rotational axis, focusing actions and further deflections before finally crossing the original beam axis in the isocenter at an obtuse angle, for example, at right angles to the rotational axis and hence to the original beam axis (compare FIG. 1). The deflections are implemented by dipole fields and the focusing actions by successive crossed quadrupole fields. To irradiate a spatially extensive tumor, the particle energy according to the depth of the tumor in the tissue may be varied and it's the energy along the two other spatial coordinates in the layer of this depth may be varied. Two scanner magnets, which are part of the gantry, can deflect the beam by a small angle in the horizontal and/or vertical plane since. In contrast to the other magnets, the scanner magnets can be driven very quickly. Adapting the current feed to the other deflection and focusing magnets in line with the respective particle energy can only be done slowly.
In one optical system, the scanner magnets are (other than shown in FIG. 1) the last ion-optic component of the gantry. The beam diameter can be kept small through the entire gantry, for example, the aperture and also the overall installation size of the previously traversed deflection and focusing magnets can be kept small. Due to the deflection caused by the scanner magnets, however, the particles strike the area to be treated at different angles. The particle beams may have a parallel incidence.
In another optical system, the scanner magnets are placed ahead of the terminating deflection magnet. Accordingly, the defocusing caused by the scanner magnets can be compensated by the following magnet, so the particles can exit the gantry in virtually parallel beams.
If magnets with iron yoke are used for the gantry, as is typically the case with ion-optic systems, the maximum magnetic flux density attainable is limited to about 2 Tesla due to saturation effects in the iron (or the ferromagnetic material used in this case). The achievable deflection radius is inversely proportional to the magnetic flux density. When ferromagnetic yoke material is used, the necessary deflection angle is only achieved with a size of magnet system that is unacceptable in terms of weight and costs. The use of superconducting air-core coils represents an alternative. Accordingly, considerably higher flux densities can be realized, with the result that the required deflection radius is reduced by the corresponding factor.
Only the final, terminating 90° deflection magnet is considered. The magnet can be implemented by individual coils with a rectangular cross-section, which enclose the volume of the particle beam. An actively shielded design includes, for example, two coils closed via the outsides and pairs of main, end and main correction coils are used, as shown in FIG. 2. The positions and cross-sections of the individual coils are usually chosen such that the ion-optic requirements for particle beams exiting in parallel and homogeneous spatial distribution of the particles are fulfilled.
When a deflection magnet is being configured, a problem that routinely occurs is that the ion-optic requirements in terms of parallelism with at the same time homogeneous spatial distribution of the particles in the plane of the isocenter (for linearly driven scanner magnets) cannot be perfectly fulfilled in principle. FIG. 3 shows an example of a spatial and angular deviation for a coil configuration. When ferromagnetic components are used, for example, for shielding individual components, the B fields are no longer linear to the current, as a result of which the imaging can only be optimal for one ion energy.
The possibilities of fulfilling the ion-optic requirements (parallelism and desired spatial distribution) are essentially limited. Limitations arise, for example, from the limited current carrying capacity of real conductors. This also applies to superconductors, whose maximum current densities continue to be heavily dependent on the magnetic flux density at the location of the conductor.
Arbitrarily high requirements in terms of parallelism with simultaneous homogeneous spatial distribution can only be achieved with very great deflection radii. This, however, conflicts with the requirement for wanting to implement as compact and lightweight a deflection magnet as possible.